1. Field of the Invention
The present invention generally relates to communication systems and, more particularly, is directed to a communication system having a reference reception carrier frequency which differs from and follows a reference transmission carrier frequency.
2. Description of the Prior Art
A TDMA (time division multiple access) system is known as one type of digital cellular communication system in which a base station and a mobile station are connected via radio waves. According to this TDMA system, six receiving slots, for example, are provided for every channel of the 900 MHz band and in each mobile station an incoming signal in one of the six slots is received for a period of 20 milliseconds at intervals of 120 milliseconds. Six transmission slots are similarly provided for every channel, and in each mobile station an outgoing signal in one of the six transmission slots is similarly transmitted. A reference receiving carrier frequency and a reference transmitting carrier frequency in a single mobile station are different from each other. By way of example, a frequency 45 MHz lower than the reference receiving carrier frequency is employed as the reference transmitting carrier frequency.
When the mobile station is moved, an offset frequency occurs in the actual received carrier frequency due to the Doppler effect. Also, if the oscillation frequency of a reference oscillator in the base station, which uses a quartz crystal, is not correct, then an offset frequency will also occur. In the mobile station, the offset frequency of the transmission carrier frequency must be maintained within a band.+-.200 Hz from the reference transmitting carrier frequency.
For this reason, it has been proposed that the mobile station be provided with a PLL (phase locked loop) which is locked to the reception carrier frequency of the received signal to thereby produce the transmission carrier signal. However, since the received signal is a burst signal which is received for 20 milliseconds at intervals of 120 milliseconds, it is very difficult to lock the PLL to the reception carrier frequency of the received signal, as the lock frequency must be determined during a short period of time (20 milliseconds) and maintained for a period of 100 milliseconds between received signals, and the transmission offset frequency must be maintained within the band.+-.200 Hz from the reference transmitting carrier frequency.